This application claims the priority of 198 42 627.5-21,filed Sep. 17, 1998.
The present invention relates to a steering system for vehicles, which can be operated in a normal mode, in which a steering handle and steered vehicle wheels are connected to one another via an electric or electronic control system (steer-by-wire level), and in an emergency mode, in which the steering handle and the steered vehicle wheels are positively coupled mechanically and/or hydraulically (fall-back level).
A two level steering system of this kind generally has a control unit which continuously monitors the functionality of the steer-by-wire components of the steering system and, in particular, checks redundantly available information for plausibility. As soon as the control unit detects a fault, it switches from normal mode to emergency mode to enable higher operational reliability to be ensured for the vehicle.
The so-called "fall-back level" of the steering system, i.e. the mechanical and/or hydraulic positive coupling which is activated in the emergency mode between the steering handle and the steered vehicle wheels, can be formed, for example, by a conventional steering line containing a clutch by way of which the mechanical steering line can be activated for emergency operation.
As an alternative to a mechanical steering line, hydraulic positive coupling via a so-called "hydraulic rod" can also be provided. A hydraulic rod of this kind has a manual-side, first piston-cylinder unit, the piston of which is displaced axially in the associated cylinder by actuations of the steering handle. Hydraulic fluid is displaced out of a chamber formed by the piston in the cylinder and fed to a wheel-side, second piston-cylinder unit. The hydraulic fluid displaced out of the first piston-cylinder unit displaces the associated piston in the second piston-cylinder unit, this piston being coupled mechanically to the steered vehicle wheels, which turn as a result.
DE 196 22 731 A1 discloses a hydrostatic steering device which has two control systems. Each of these control systems has a hydraulic pump, a servo valve and a servomotor. The servo valves are actuated by a common steering wheel. The known steering device furthermore has a change-over valve which, in its operating position, hydraulically connects the motor lines of the second control system and, in an emergency position, hydraulically connects the motor lines of the first control system. Also provided is an actuating device which responds when a fault occurs in the first control system and switches the change-over valve from the operating position into the emergency position. With the known steering device. Therefore, servo-assisted steering is possible even if the first control system fails. A steering device of this kind, which has two complete, separately operating control systems, is relatively expensive.